The present invention relates to imaging, and more particularly, to a color filter array.
A simplified cross sectional view of an imaging system is illustrated in FIG. 1. Optical system 102 focuses electromagnetic radiation onto a focal plane, which may be taken as Color Filter Array (CFA) 104 and pixel sensor array 106. The CFA is usually deposited over pixel sensor array 106 by photo-lithographic techniques well known to the semiconductor industry. Pixel sensor array 106 is an array of pixel sensors, where in general, a pixel sensor is any sensor which absorbs radiation and provides a signal indicative of the absorbed radiation. Pixel sensor array 106 may be, for example, an array of charge coupled devices (CCD), or an integrated array of CMOS (Complementary Metal Oxide Semiconductor) pixel circuits. A pixel circuit may comprise a photo diode, where photons absorbed by the photo diode generate electron-hole pairs, along with additional circuits to provide an electrical signal, either a voltage or current signal, indicative of the number of photons absorbed by the photo diode. Photons incident upon various pixel circuits are pictorially indicated by 108.
The spectral content of electromagnetic radiation focused onto a focal plane depends upon, among other things, the imaged subject, the illumination of the subject, the transmission characteristics of the propagation path between the imaged subject and optical system 102, the materials used in optical system 102, as well the geometric shape and size of optical system 102. For consumer imaging systems, the spectral range of interest is the visible region of the electromagnetic spectrum.
The CFA is an array of filters, usually contiguous and deposited over pixel sensor array 106 so that each pixel sensor is substantially sensitive to only the electromagnetic radiation passed by one filter. (A filter in the CFA may actually be a composite filter manufactured from two or more filters, so that the transfer function of the resulting filter is the product of the transfer functions of its constituent filters.) Each filter in the CFA passes electromagnetic radiation within some spectral range. For example, common color filter arrays include a RGB (Red-Green-Blue) CFA, composed of red, green, and blue filters only, a CMY (Cyan-Magenta-Yellow) CFA, composed of cyan, magenta, and yellow filters only, or a CMYG CFA (Cyan-Magenta-Yellow-Green), composed of cyan, magenta, yellow, and green filters only. The pattern of filters making up an CFA is often a tiling pattern, where the tiling pattern is a repetition of a simpler, smaller pattern of filters, which we term the unit cell array.
The color fidelity of color CMOS and CCD imaging sensors may be highly dependent upon the selection of CFA materials, as well as the CFA tiling pattern. The RGB CFA is often selected for its simplified signal processing requirements and good color fidelity. The CMY and CMYG color filter arrays are often selected for use in applications which require improved light sensitivity. However, the signal-to-noise ratio of imaging sensors with CMY or CMYG color filter arrays is often less than that of imaging sensors with a RGB CFA.
It is therefore desirable to provide a CFA for imaging sensors so that there is both good color fidelity and relatively high signal-to-noise ratio.